Are mitochondria their own species?

[Blalron]

Unlike other organelles, they have their own separate DNA and reproduce independently of the cells they are found in.

So aren’t we and mitochondria basically two species living in a symbiotic relationship?

[Darwin_s_Finch]

I wouldn’t go so far as to qualify mitochondria as a separate species, but there are certainly some (e.g., Lynn Margulis) who believe the origins of eukaryotes can be traced to just such a primeval symbiosis:

[ul]
[li]Mitochondria have a set of double membranes, as do most bacteria; and the lipid composition of mitochondrial membranes shows no similarities with the eukaryotic cell cytoplasm. If mitochondria evolved inside of proto eukaryote, their membrane composition would be expected to be of the same material. Instead, it appears that mitochondrial membranes more closely resemble bacterial membranes in terms of lipid composition (Margulis, 1981, p. 217). [/li][li]The inner-membrane infoldings in the mitochondria lend more credence to the symbiosis theory. According to Margulis, the infoldings (cristae) “are adaptations that increase the surface area of oxidative enzymes, evolutionary analogues to the mesosomal membranes of many prokaryotes” (Margulis, 1981, p. 208). In addition, the cristae keep the various enzymes segregated according to use, as do bacteria. [/li][li]Mitochondrial ribosomal RNA sequences bear much more in common with bacteria than with ribosomes in the eukaryotic cytoplasm. For example, n-formylmethionyl transfer RNA has been found to exist only in mitochondria and bacteria (Dyer and Obar, 1985, p. 78). [/li][li]Not only do mitochondria possess their own DNA, but it is circular, as is bacterial DNA; and DNA synthesis is continuous as opposed to that of eukaryotic nuclear DNA. Furthermore, the ratio of guanine-cytosine base pairs in mitochondrial DNA is proportionately higher, as with bacteria, than in eukaryotic nuclear DNA (Margulis, 1981, p. 206). [/li][li]Mitochondrial division resembles bacterial reproduction, according to Margulis. She writes, “Genetic recombination in (mitochondria) is far more reminiscent of phage and bacterial sexuality than it is of eukaryotic nuclear sexual behavior” (Margulis, 1981, p. 218).[/ul][/li] A closer look at mitochondrial ribosomes reveals even more supportive evidence for symbiosis theory. Mitochondrial ribosomes have more similar antibiotic sensitivities with bacterial ribosomes than with eukaryotic ribosomes. For example, cycloheximide blocks eukaryotic ribosomes by affecting tRNA transfer but affects neither mitochondria or bacteria. On the other hand, drugs that block prokaryotic synthesis but not eukaryotic synthesis block mitochondrial protein synthesis as well, e.g., erythromycin and tetracycline (Margulis, 1981, p. 217-218).

http://cas.bellarmine.edu/tietjen/images/origin_of_mitochondria_in_eukary.htm

[Bob_Scene]

Mitochondria certainly were a separate species a long time ago, before they colonized primitive eukaryotic cells. The endosymbiotic origin of mitochondria and certain other organelles has been extremely well established in recent years. (Note that the cites above are from 1981, and things have changed drastically since then. Margulies wrote all that before the advent of molecular phylogenetic analysis, which gives very powerful additional evidence for the endosymbiotic model.)

But it’s been an extremely long time since we entered into this symbiotic relationship with mitochondria, and most of the proteins they need to function are now encoded by the nuclei of the cells they inhabit. Biologists consider mitochondria now just to be part of eukaryotic cells, and not a separate species.

Note, though, that “species” is an artificial concept, and the answer to a question like this, and a lot of other questions about species, is often going to be a matter of opinion.